There are numerous sites on rivers, in estuaries, and in regions of strong off-shore currents, where substantial energy could be extracted from the flowing water, but where a dam is impractical to build. What is needed is a practical water turbine that can operate in an open stream, i.e., at very low pressure head. Conventional water turbines generally require a much higher pressure head than is available in open streams, and thus require dams.
There have been prior efforts at open stream turbines. Most familiar, of course, is the paddle-wheel type in which the drag force on the paddles turns the turbine. Numerous designs have been proposed: some fully submerged, others not; some with vertical axes of rotation, others with horizontal axes. Some such designs are centuries old, but there has continued to the present day a strong interest in paddle-type water turbines, e.g., Gutsfeld (U.S. Pat. No. 4,104,536), Kurakake (U.S. Pat. No. 4,095,422), Antogini (U.S. Pat. No. 3,976,396), and Herz (U.S. Pat. No. 3,922,012). All such designs, however, have very low efficiency (i.e., very little of the available power in the stream is actually extracted) owing to their dependence on drag.
There have been some efforts at using lifting surfaces in open stream water turbines. Darrieus U.S. Pat. No. 1,835,018 suggests a turbine that has a plurality of vertical, airfoil-like blades supported at the top and bottom for rotation around a central vertical axis. Bailey U.S. Pat. No. 3,978,345 shows several water turbines strung across a stream; the axis of rotation is horizontal and the blades, support structure and power take-off equipment are for the most part below water. Both the Darrieus and Bailey turbines have axes of rotation that are transverse to the flow direction, and thus are known as cross-flow turbines.
To maximize the efficiency of a cross flow turbine it is known that blade pitch should vary cyclically, to maintain an angle of attack that maximizes lift over drag. Darrieus shows a common eccentric ring and associated links to cyclically pitch the blades. The Bailey open stream turbine uses a central hydraulic control and hydraulic lines running therefrom to the blades to cyclically change the attitude of control vanes on the trailing edges of the blades. Both arrangements thus depend on a central control unit to prescribe pitch for all blades.